BAGHDAD, Iraq (CNN) -- Thirty Marines and a Navy corpsman were killed in a helicopter crash near Iraq's border with Jordan, bringing the number of U.S. troops killed Wednesday to 37 -- the deadliest day for U.S. forces since the start of the war in Iraq.

Four U.S. Marines were killed during combat in Iraq's Anbar province, and two U.S. soldiers were killed in attacks in the Baghdad area, according to the U.S. military. (Most based in Hawaii)

The cause of the chopper crash is under investigation, although it appears to be weather-related, according to Gen. John Abizaid, chief of U.S. Central Command.

"The weather was bad. We don't know of any enemy action; the investigation continues," he said.

There was no evidence of any survivors, according to Abizaid, who said the helicopter was on "a routine mission in support of the elections."

"It's a dangerous environment that we operate in Iraq, we all understand that, and again our condolences to the families," he said.

Wednesday's death toll surpassed the 31 U.S. troops killed on March 23, 2003 -- four days after the start of the war in Iraq. Twenty-nine of them died in combat that day.

Wednesday's incidents brought the U.S. death toll in the war to 1,418 -- 1,085 of them in hostile action.

TheCH-53E Super Stallioncrashed about 1:20 a.m. local time (5:20 p.m. Tuesday ET) near Ar Rutba in Anbar province in western Iraq.(Map)

It was carrying personnel from the 3rd Marine Aircraft Wing and the 1st Marine Division.

The four other Marines who died Wednesday were killed during combat operations elsewhere in Anbar province, according to a military news release. The Marines were assigned to the 1st Marine Expeditionary Force.

A U.S. soldier was killed Wednesday when insurgents attacked a combat patrol with grenades near Ad Duluiyah, military officials said.

The soldier, from the 1st Infantry Division, died and two others were wounded in the attack about 11:20 a.m. (3:20 a.m. ET). The injured were taken to a military hospital for treatment; one was in serious condition.

A roadside bomb killed another U.S. soldier and wounded two others Wednesday afternoon in the southern Baghdad district of al-Rashid, according to the 1st Cavalry Division.

President Bush said, "Any time we lose life it is a sad moment," at a news conference Wednesday, referring to the deaths in Iraq. (Full story) (Transcript)

In addition to the U.S. military deaths, four multinational soldiers were wounded Wednesday morning when a car bomb exploded near a convoy in southwestern Baghdad along the road to the airport, a source with the 1st Cavalry Division said.

Seven Iraqis killed

In Tamin province, also on Wednesday, three car bombs within an hour killed five Iraqis and injured six other people, according to the police chief in Kirkuk.

The first was in the town of Riyadh, about 30 kilometers (19 miles) west of Kirkuk, and targeted a police station, he said. Three Iraqi police officers were killed and three civilians injured.

The second detonated outside the Riyadh mayor's office, killing two Iraqi soldiers. The third bomb exploded outside Riyadh and targeted a U.S. military convoy. Three other Iraqi civilians were wounded.

Insurgents attacked the offices of two political parties in Baquba on Wednesday, triggering clashes that left an Iraqi police officer dead and four others wounded -- three of them working as guards for the parties, Baquba police said.

The insurgents used grenades and small arms fire to attack the Kurdish Democratic Party office and the office of the Iraqi Patriotic Gathering Alliance about 6:30 a.m. (10:30 p.m. Tuesday ET), police said, and the resulting battles lasted two hours.

In al-Nahrawan, a southeastern suburb of Baghdad, city council leader Karim Sarhan was gunned down Wednesday morning in a drive-by shooting on his way to work, Iraqi police said.

U.N. sees basis for credibility

U.N. officials Wednesday called conditions for balloting in Iraq "far from ideal" but said they are satisfied that Iraq has set up a basis for credible elections.

Iraqis go to the polls Sunday to pick a transitional national assembly. Insurgents have stepped up attacks, targeting polling stations and warning candidates and would-be voters that their lives will be at risk.

Carina Perelli, chief U.N. electoral adviser, praised the Iraqi electoral commission for its work, saying, "From a technical point of view these elections are as sound as they can be under the circumstance."

"Iraqi citizens are faced with a very tough position of basically having to confront their fears," Perelli said. They must "decide for themselves whether they consider that this election is important enough ... in order to risk their lives to go and vote."

U.N. Political Affairs Under-Secretary Kieran Prendergast said, "Conditions are far from ideal. Security is a problem. We know that some Iraqis don't feel ready."

But Prendergast said, "None of this is surprising. Iraq is emerging from an extremely traumatic chapter in its history."

Asked about the U.S. military handing out fliers and urging people to vote, Perelli said she had been "asking, begging military commanders precisely not to do that."

"They should try to minimize their participation because this is an Iraqi process. It's not a U.N. process," she said.

Wednesday night, a spokesman for U.N. Secretary-General Kofi Annan said Perelli "misspoke" when she said the U.S. military was over-enthusiastic in wanting to help with the Iraqi elections.

And Abizaid said the mission of U.S. troops is to provide security, not spur turnout.

"American soldiers do not have the mission to get the vote out," he said.

In Washington, the chairman of the Senate Armed Services Committee predicted Iraq will face a post-election period of uncertainty, during which insurgent attacks will likely go up.

"The aftermath of the elections is fraught with uncertainty," said Sen. John Warner, a Virginia Republican. "We should prepare the American people for as many eventualities as could possibly happen."